Draw a line beneath the curve of the coving, on the wall. Use a laser level, or an ordinary spirit level, to get the line accurate.
Place painter's masking tape beneath the line along the length of the wall.
Repeat the process to draw another straight line at the top of the coving curve if you want to paint the curve a color that's different than the wall and the ceiling. If you want the curve the same color as the ceiling, skip this step.
Paint above the painter's tape and across the ceiling for an all-one-color ceiling and coving. Paint just the coving if you want the coving a different color.
Wait for the paint to dry, then remove the tape. Re-stick masking tape on the other side of the line to keep the line straight while you paint on the wall below it.
Allow all the paint to dry thoroughly and then remove the tape.